BOG (RBBP-9) is a novel cellular protein which interacts with pRb, p107 and p130 through the LXCXE motif. The homozygous BOG-/- mouse has been generated, and no detectible level of BOG mRNA nor protein was found in the homozygous animals. BOG-/- are born healthy, grow at a normal rate compared with BOG+/- and wildtype animals, and are fertile. At present we are performing histological evaluations of numerous haematoxylin and eosin stained tissues from BOG-/- mice. We have started to use mouse embryo fibroblasts (MEFs) isolated from 13.5 day old BOG -/- embryos in search of a role for BOG in cell cycle control, following standard procedures (3T3 and 3T9 protocols) and used for in vitro experiments. Preliminary results indicate that BOG-/- MEFs possess a more limited capacity to proliferate than the wildtype (WT) cells. The -/- cells stopped proliferating sooner than the WT resulting in a slow but progressive decrease in the total number of cells. The results were identical under both high and low cell density conditions. The WST-1 based proliferation assay was repeated using two new lines. The results confirm that the impared growth capacity of BOG-/- MEF is directly related to the genotype. Taking together the data obtained with the proliferation assays suggested that BOG null MEFs exhibited a "premature senescence" phenotype. To test that hypothesis, we chose the senescence associated b-galactosidase activity as a senescence marker. The results were consistent with an earlier acquisition of a replicative senescence status in the BOG deficient cells.